Mina Dawood

Bond Behavior of CFRP Strengthened Steel Structures

Recent research has focused on rehabilitation and strengthening of steel structures and bridges using fiber reinforced polymer (FRP) materials. The bond behavior of FRP materials to steel structures is quite different from that of concrete structures. Preliminary test results showed the occurrence of very high bond stresses for most strengthening applications due to the amount of strengthening required for steel structures and bridges. In this paper, surface preparation methods and means of preventing galvanic corrosion are discussed. The results of an experimental program for selection of suitable adhesives through determination of the development length is discussed as well as preliminary testing showing the importance of proper detailing of the ends of the FRP strips. The shear stress distribution determined in the experimental program is compared to analytical models using a stress-based approach. The remainder of the paper focuses on the current methods for determining bond stresses and their use for the design of FRP strengthening system for steel structures.

Flexural Strength Design of Concrete Beams Reinforced with High-Strength Steel Bars

Current design requirements limit the allowable design strength of high-strength steel reinforcements to 80 ksi (550 MPa), which limits engineers from fully using the enhanced strength characteristics of low-carbon and chromium high-strength steel. In this paper, a methodology is presented for the flexural strength design of concrete beams reinforced with high-strength reinforcing steel that conforms to the requirements of ASTM A1035-07. The design method is based on simple analysis techniques that satisfy fundamental principles of equilibrium and compatibility. Strain limits for tension-controlled sections and compression-controlled sections are proposed that are consistent with the approach of the current and past ACI 318 Codes. The proposed method is compared with previously-reported experimental results, and the application of the proposed method is demonstrated by a numerical design example. Findings indicate that flexural members designed using the proposed design methods and criteria have comparable flexural strength characteristics with members designed according to current ACI 318 requirements, using Grade 60 (400 MPa) and Grade 75 (520 MPa) reinforcing steel.

Global and Local Fiber Optic Sensors for Health Monitoring of Civil Engineering Infrastructure Retrofit with FRP Materials

Fiber-reinforced polymer (FRP) materials are currently used for strengthening civil engineering structures and bridges. The effectiveness of the strengthening system is highly dependent on the bond characteristics of the FRP material to the external surface of the structure. This article presents the application of two types of fiber optic sensors, which can be embedded in FRP materials to monitor the global and local behavior of the strengthened structure, respectively. The global sensor is designed to evaluate the overall condition of a structure based on the measured elongation of the FRP layer along the entire span of the structure. The success of this low-cost global sensor has been demonstrated using a full-scale prestressed concrete bridge girder that was loaded up to failure. The test results indicate that this type of sensor can be used to identify major changes in the overall behavior of the structure such as cracking of prestressed members or yielding of the internal reinforcement. The second sensor component consists of fiber Bragg grating sensors. The sensors were used to monitor the behavior of steel double-lap shear specimens tested under tensile loading up to failure. The measurements were used to identify abnormal structural behaviors such as epoxy cracking and/or FRP debonding. The test results compared well to the numerical values obtained from a three dimensional shear-lag model that was previously developed to predict the sensor response.

Development of a self-stressing NiTiNb shape memory alloy (SMA)/fiber reinforced polymer (FRP) patch

The objective of this research is to develop a self-stressing patch using a combination of shape memory alloys (SMAs) and fiber reinforced polymer (FRP) composites. Prestressed carbon FRP patches are emerging as a promising alternative to traditional methods to repair cracked steel structures and civil infrastructure. However, prestressing these patches typically requires heavy and complex fixtures, which is impractical in many applications. This paper presents a new approach in which the prestressing force is applied by restraining the shape memory effect of NiTiNb SMA wires. The wires are subsequently embedded in an FRP overlay patch. This method overcomes the practical challenges associated with conventional prestressing. This paper presents the conceptual development of the self-stressing patch with the support of experimental observations. The bond between the SMA wires and the FRP is evaluated using pull-out tests. The paper concludes with an experimental study that evaluates the patch response during activation subsequent monotonic tensile loading. The results demonstrate that the self-stressing patch with NiTiNb SMA is capable of generating a significant prestressing force with minimal tool and labor requirements.

Effective Splices for a Carbon Fiber–Reinforced Polymer: Strengthening System for Steel Bridges and Structures

Carbon fiber–reinforced polymer (CFRP) materials have been used successfully to strengthen reinforced concrete bridges and structures. Recently, a new high modulus CFRP strengthening system was developed to increase the allowable load carrying capacity and to enhance the serviceability of steel bridges and structures. Because of the relatively high flexural rigidity of the CFRP materials, the length of the CFRP plates that can be transported to the job site is limited. To implement the proposed strengthening system in longer-span steel bridges, adjacent lengths of CFRP must be spliced. To develop an effective splice joint for the proposed strengthening system, an experimental and analytical research program was conducted to study the bond behavior of the CFRP materials. The parameters considered included plate end geometry, splice length, and the possibility of using mechanical anchorage. The analytical study included a finite element analysis to determine the distribution of the stresses within the adhesive layer for different splice configurations. On the basis of the findings, a simplified method was proposed to design lap splice joints with different reversed taper angles and adhesive properties. The research concluded that, with proper detailing, the proposed CFRP system could be effectively used to strengthen steel bridges and structures.

Innovative Use of FRP for the Precast Concrete Industry

This paper presents several advancements in the use of fiber reinforced polymer (FRP) materials for the precast concrete industry. Precast concrete members are commonly selected for reasons such as the high level of quality control used in their production, the durability of the finished structure, reduced labor costs and shorter construction schedules, and the economics of scale achieved with mass-production of components. The environmental durability, high strength to weight ratio, and ease of installation of FRP reinforcements an attractive alternative material for the precast industry. This paper presents several advancements in the use of FRP grid as flange reinforcement for precast double-tee members, as a shear transfer mechanism for thermally efficient composite and partially-composite load bearing wall panels, as reinforcement for precast architectural cladding panels. Each of these applications highlights the advantages of using FRP materials to achieve significant enhancement of the structural, thermal and architectural performance. The innovative use of the FRP materials and the unique construction techniques described have resulted in the development of safe and functional structures, as demonstrated by the research conducted by the authors and others in collaboration with the precast industry.

Fatigue Life Assessment of Cracked High-Mast Illumination Poles

AbstractThis paper presents the findings of a research program that was conducted to evaluate the probability of failure of high-mast illumination poles (HMIPs) with preexisting cracks at the pole-to-base plate connection detail. A simplified reliability-based analysis framework is presented to evaluate the safe service life of the cracked HMIPs. Vibrations induced by both vortex shedding and natural wind gusts are considered. The influence of different parameters is assessed, including pole geometry, pole location, terrain effects, local wind characteristics, and the assumed level of fatigue degradation attributable to the presence of a preexisting crack. The safe service lives of different standard HMIP designs that are commonly used by the Texas DOT are calculated. The results indicate that the degradation of the poles is predominantly attributable to vortex shedding–induced vibrations in the second and third vibration modes. The findings also demonstrate that the safe service life of a HMIP depends pr...

Behavior and Performance of Fiber-Reinforced Polymer-to-Steel Bond

This paper presents a detailed discussion of bond behavior and fatigue, which have a major, often overlooked, impact on the long-term performance of steel bridges strengthened with fiber-reinforced polymer (FRP) materials. The paper discusses the primary factors that affect the bonding behavior between steel and FRP materials. The mechanisms of bonding are highlighted, and the main factors that contribute to the environmental degradation of bonds are outlined. Techniques to produce high-quality, durable, and reliable bonds are presented; these techniques include surface preparation techniques, use of primers, and use of various plate end details to reduce bond stress concentrations and mitigate debonding. Various factors that affect the fatigue performance of FRP materials and structural adhesives are enumerated, and the behavior of FRP patches used to repair cracked steel members is described. This paper highlights some complexities and nuances associated with the design and installation of an FRP-based retrofit system for steel bridges and structures and some key practices that should be adopted to ensure the effective and reliable performance of the retrofit.

Experimental Investigation of Bond between High-Modulus CFRP and Steel at Moderately Elevated Temperatures

AbstractThis paper presents the findings of a research study that was conducted to assess the effect of moderately elevated temperatures, up to 50°C, on the bond behavior of steel beams strengthened with externally bonded high-modulus carbon-fiber-reinforced polymer (CFRP) plates. In the first phase of the testing, seven steel-CFRP bonded double-lap shear coupons were tested at different temperatures to characterize the bond behavior. In the second phase of testing, steel beams were strengthened with different lengths of high-modulus CFRP plates and loaded under four-point bending at 25, 40, and 50°C. The test results indicate that the debonding load of bonded joints at 50°C is between 56 and 116% greater than the debonding load of similar joints at 25°C for the specific system tested in this study. This increase was attributed to an increase of the toughness of the tested adhesive at 50°C despite the lower tensile strength and stiffness of the material. The research findings suggest that the proposed str...

Durability of steel components strengthened with fiber-reinforced polymer (FRP) composites

Abstract: Fiber-reinforced polymer (FRP) materials are gaining prominence as an alternative to traditional materials and methods for repair and rehabilitation of existing steel structures. This chapter describes the primary environmental degradation mechanisms of steel structures that are strengthened with externally bonded FRP materials with an emphasis on carbon FRP (CFRP). Specifically, galvanic corrosion, degradation of the steel–adhesive interface and degradation of the bulk adhesive are discussed in detail. The effects of thermally-induced stresses and exposure to moisture are summarized. Best practices to enhance the environmental durability of FRP-strengthened steel members are also presented. Overall, this chapter indicates that, based on an understanding of the predominant environmental degradation mechanisms, FRP systems can be designed and detailed to provide an effective long-term system for the repair and rehabilitation of existing steel structures.